Actress Quits 'Roseanne' Over Tweet from Barr

Wanda Sykes is following Whitney Cummings out the door of "Roseanne" -- but her exit is a direct response to Roseanne Barr's racist attack on an Obama aide, a woman who's a close friend of Wanda's ... TMZ has learned.

Wanda kept it short and sweet, saying ... "I will not be returning to ['Roseanne']." The timing is no coincidence, coming within an hour of Barr's apology for calling an Obama adviserthe product of "muslim brotherhood & planet of the apes."

A source close to Wanda tells us the tweet is "100 percent why she quit." We're told Wanda and Valerie Jarrett, the Obama adviser, are friends ... and were hanging out as far back as 2009, when Wanda hosted the White House Correspondents' Dinner.

Wanda was a consulting producer on the hugely successful reboot, which is returning for a second season -- but, clearly, with some producing changes.